Vehicle transmission

ABSTRACT

A vehicle transmission having a housing within which is disposed a multi-ratio main change speed gear set (X) and a first range change gear set (Z) in series and in which the speed and range changes are effected by selector forks (100, 101, 102) which are mounted on and axially movable relative to a single selector fork rail (105) which is held stationary (108) relative to the housing. Additionally selector forks (103, 104) for a second range gear set (Y) and a creeper gear facility (C) may be mounted on and movable relative to the same selector fork rail (105). The housing is also provided with an access aperture (132) and the selector fork rail (105) can be released and slid axially within the housing to allow removal of the selector forks (100, 101, 102, 103) one by one from one end of the rail (105) via the aperture (132).

TECHNICAL FIELD

This invention relates to vehicle transmissions and in particular toselector fork arrangements used to select the operative ratio of suchtransmissions.

There is a longstanding requirement for a simple and easily accessibleselector fork arrangement suitable for the type of transmission which isused in agricultural and industrial tractors (or similar vehicles) andwhich has a multi-ratio main change speed gear set and a range changegear set in series.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide a vehicletransmission of the above described type which has an improved selectorfork arrangement.

Thus according to the present invention there is provided a vehicletransmission having a housing within which is disposed a multi-ratiomain change speed gear set and a first range change gear set in seriesand in which the speed and range changes are effected by selector forkswhich are mounted on and axially movable relative to a single selectorfork rail which is held stationary relative to the housing.

The transmission may have a second range change gear set operated by afurther selector fork which is also mounted on and movable relative tothe same selector fork rail.

Additionally the transmission may include a creeper gear facility whichis operated by a still further selector fork which is also mounted onand movable relative to the same selector fork rail.

Preferably the housing is provided with an access aperture and saidselector fork rail is releasably mounted in the housing in a mannerwhich permits the rail to be axially slid within the housing after therelease of rail fastening means to allow removal of the selector forksone by one from one end of the rail via said aperture. Such anarrangement enables the selector forks to be easily accessible andremovable from the housing.

One of the range change selector forks may be moved axially relative tothe rail by an hydraulic cylinder which surrounds the rail.

Preferably the hydraulic cylinder is double-acting and contains adifferential area double-acting piston which surrounds the rail anddivides the cylinder into two operating chambers, the arrangement beingsuch that if both operating chambers are subjected to the same level ofpressure the piston moves in a first direction and if only one chamberis pressurised the piston moves in the opposite direction.

The invention also provides a vehicle transmission having a housingwithin which is disposed a multi-ratio change speed gear set in whichthe speed changes are effected by selector forks which are mounted onand axially movable relative to a single selector fork rail which isheld stationary relative to the housing, the housing being provided withan access aperture and the selector fork rail being releasably mountedin the housing in a manner which permits the rail to be axially slidwithin the housing after the release of the rail fastening means toallow removal of the selector forks one by one from one end of the railvia said aperture.

The invention also provides a selector fork arrangement for a vehicletransmission contained within a housing, the arrangement comprising aselector rail, selector rail fastening means for holding the railstationary relative to the tansmission housing, and a plurality ofselector forks axially movable relative to the rail to effect changes inthe operative ratio of the transmission.

DESCRIPTION OF THE DRAWINGS

One embodiment of the present invention will now be described by way ofexample only, with reference to the accompanying drawings in which:

FIG. 1 is a side view of a tractor transmission which is controlled bythe selector mechanism of the present invention;

FIG. 2 diagrammatically shows the gear wheel layout of the input gear Zof the transmission;

FIG. 3 is a plan view of the selector fork layout of the transmission;

FIG. 4 is an enlarged side view in the direction of Arrow IV of FIG. 3of the hydraulic High/Low range actuating cylinder;

FIG. 5A is a scrap plan view in the direction of Arrow VA of FIG. 5;

FIGS. 5, 6 and 7 are views of the gear lever selector mechanism in thedirections of Arrows V, VI and VII of FIG. 3 respectively;

FIGS. 8 and 9 show the operation of the interlock used in the main gearset X;

FIGS. 10 and 11 diagrammatically illustrate the movement of gear lever120 (FIG. 10 being a corresponding view to FIG. 5 and FIG. 11 being inthe direction of Arrow XI of FIG. 5);

FIG. 12 and 13 show the operating gates for gear levers 120 and 121respectively;

FIGS. 14 and 15 show side and end views respectively of the selectormechanism of input gear Z in the directions of Arrows XIV and XVrespectively shown in FIGS. 15, 14 and 3;

FIGS. 16 to 19 show diagrammatic plan views of the various movements ofselector members 153 and 157 and selector shaft 113;

FIGS. 20 to 23 show diagrammatically the various movements of gear lever121 involved in the selection of all four ratios of input gear Z (FIGS.20 and 22 correspond to FIG. 7 and FIGS. 21 and 23 are in direction ofArrow XXI of FIG. 7);

FIGS. 24 and 25 show in tabular form the gear wheels used to engage theforward and reverse ranges of the transmission;

FIG. 26 shows a modified form of gear lever selector mechanism;

FIG. 27 shows a view in the direction of Arrow XXVII of FIG. 26, and

FIG. 28 shows in section an alternative form of gear lever mount.

The transmission (shown in FIG. 1) comprises a main four-speed changespeed gear set X, a two-speed High/Low output gear set Y, and a constantmesh input gear set Z capable of providing two forward ratios and tworeverse ratios.

BEST MODE OF CARRYING OUT INVENTION

The main change speed set X comprises four gears 10, 11, 12 and 13 whichare splined onto a lay shaft 14 and four constant mesh gears 15, 16, 17and 18 which are rotatably mounted on the two portions 19 and 20 of thetransmission output shaft.

Gears 15 and 16 are associated with synchromesh unit 21 which has aslidable sleeve O and which provides first and second ratios by slidingsleeve O to the left and right respectively while gears 17 and 18 areassociated with synchromesh unit 22 which has a slidable sleeve P andwhich provides third and fourth ratios by sliding P to the left andright respectively.

The output gear set Y comprises a gear Q slidable on splines 24 providedwith a gear 25 which rotates with the lay shaft 14 to provide a lowoutput train ratio. By sliding the gear Q on the splines 24, teeth 26 onthe gear Q can be meshed with co-operating teeth 27 on the gear 18 thuslocking the gear 18 to the shaft portion 20 and providing a higheroutput train ratio via gears 13 and 18.

The input gear set Z comprises gears 28 and 29 which are rotatablymounted on an input shaft 30. These two gears are in constant mesh withgears 31 and 32 which are in turn rotatably mounted on the output shaftportion 19. Mounted behind the gears 28, 29, 21 and 32 as viewed in FIG.1, are two idler gears 33 and 34 shown diagrammatically in FIG. 2 whichmeash with gears 32 and 28 respectively. The idler gears rotate with acommon shaft 35.

Associated with the gears 28 and 29 is a syncromesh unit whose sleeve Mcan be used to couple either the gear 28 or the gear 29 to the inputshaft 30. A simple sliding collar N is used for coupling either the gear31 or 32 to the output shaft 19 although a further synchromesh unitcould be used if desired.

The input gear set Z can be arranged to provide a high forward ratio bymoving the sleeve to the left to couple the gear 28 to the input shaft30 and also sliding the collar N to the left to contact the gear 31 withthe shaft portion 19. A low forward ratio can be provided by engagingthe sleeve with the gear 29 and sliding the collar N to the right tocouple the gear 32 with the shaft portion 19.

A first reverse ratio is obtained from the input shaft Z when the sleeveM couples the gear 28 to the input shaft 30 and the collar N couples thegear 32 to the shaft portion 19. With the synchronising sleeve and thecollar in these positions drive passes from the input shaft 30 via gear28, gear 34, shaft 35, gear 33 and gear 32 to the shaft portion 19. Afurther reverse ratio is obtained by engaging the synchronising sleeve Mwith the gear 29 and moving the collar N to couple the gear 31 with theshaft portion 19. In this condition drive is transmitted from the inputshaft 30 via gear 29, gear 32, gear 33, shaft 35, gear 34, gear 28 andgear 31 to the shaft portion 19.

Thus the input set Z, is capable of providing two forward ratios and tworeverse ratios which enable the whole transmission to provide, ifrequired, sixteen forward ratios and sixteen reverse ratios.

The sixteen forward ratios of the transmission are split into fourranges FA, FB, FC and FD with each of the four ratios of the main changespeed set X being engageable in each range.

Range FA is engaged when the drive through the input train Z is viagears 29 and 32 and the drive through output train Y is via gears Q and25.

Range FB is engaged when the drive through input train Z is via gears 28and 31 and the drive through output train Y is via gears Q and 25.

Range FC is engaged when the drive through input train Z is via gears 29and 32 and the drive through output train Y is via gears 13, 18 and Q.

Range FC is engaged when the drive through input train Z is vias gears28 and 31 and the drive through output train Y is via gears 13, 18 andQ.

The gear wheels used to engage each of the sixteen forward ratios aresummarised in tabular form in FIG. 24.

In a similar fashion the sixteen reverse ratios of the transmission aresplit into four ranges RA, RB, RC and RD.

The gear wheels used to engage each of the sixteen reverse ratios aresummarised in tabular form in FIG. 25.

As will be appreciated from the above, the operative forward of reverserange of the transmission is determined by the drive path which isselected in gear trains Z and Y.

If desired a two-speed planetary creeper unit C can be installed at theoutput end of shaft 20 as shown diagrammatically by dotted detail C inFIG. 1. This creeper unit doubles the number of ratios which thetransmission can provide an may, for example, have an axially slidablemechanical coupling sleeve D, which changes the operative creeper ratio.

Sleeves N,O,P Gear Q and Sleeve D are axially displaced by selectorforks 100,101, 102, 103 and 104 respectively shown in FIG. 3 which aremounted on a single fixed selector fork rail 105 which is supported at106 and 107 and held stationary in the transmission housing.

The selector forks and rail 105 are located behind the gears 31, 32, 15,16, 17, 18 and Q as viewed in FIG. 1 as can be seen from FIG. 3 which isa plane view.

Rail 105 is held fast in the transmission casing by a set screw 108 andthe position of selector forks 100, 101 and 102 relative to the rail ismaintained by detent devices 109, 110 and 111 respectively.

The axial position of forks 101 and 102 on rail 105 is controlled usingselector shaft 112 as will be explained below. A further selector shaft113 controls the axial position of fork 100 on rail 105 and also theposition of the forward and reverse selecting sleeve M of input gear setZ.

The axial position of selector fork 103 is controlled by an hydrauliccylinder 114 (FIGS. 3 and 4) which surrounds rail 105. Cylinder 114contains a differential area double-acting piston 115 which is supportedadjacent one end of a sleeve 116 which in turn surrounds rail 105 and isconnected with selector fork 103. Piston 115 is shown above rail 105 inFIG. 3 in its extreme right-hand (low range) position and below rail 105in its extreme left-hand (high range) position.

Both ends of cylinder 114 are provided with hydraulic fluid inlets 117and 118 respectively. The piston 115 and sleeve 116 are moved to theleft-hand position by connecting both inlets 117 and 118 with hydraulicfluid at the same pressure level. Movement of piston 115 and sleeve 116to the left occurs by virtue of the differential area of piston 115which, as can be seen from FIG. 4, has a larger cross-sectional areaexposed to the fluid pressure supplied via inlet 118 than via inlet 117.To move piston 115 and sleeve 116 to the right, inlet 118 isdisconnected from the pressure supply and vented so that the pressuresupplied via inlet 117 moves the piston and sleeve to the right.

This simple hydraulic piston and cylinder arrangement has the virtuethat the hydraulic circuit which supplies inlets 117 and 118 is onlyrequired to supply one level of pressure and movement of the piston 115can be effected by either simply connecting or disconnecting inlet 118from the pressure source using, for example, a solenoid operated valvewhich also vents inlet 118 when disconnecting from the pressure source.

The selector fork 104 which controls the operation of creeper unit C canbe operated either mechanical using a separate selector lever andmechanical linkage or electro hydraulically using a solenoid-operatedhydraulic valve (operated from a push button) which controls a hydraulicpiston and cylinder unit connected with the selector fork.

The selector forks 100, 101, 102 and 103 can be removed from thetransmission housing via access opening 132 after the removal of plate131 by undoing set screw 108 and then sliding rail 105 to the right asviewed in FIG. 3 and removing of plate 131 by undoing set screw 108 andthen 105 and out through opening 132 as the rail is slid to the right.The backaxle housing which is secured to the rear of the transmissionhousing is dimensioned to allow the rail 105 to be slid to the right asdescribed above.

As indicated above, sleeves O and P are controlled using selector shaft112, and sleeves M and N are controlled using selector shaft 113.Selector shafts 112 and 113 are in turn moved by gear levers 120 and 121respectively. Since the connection between the gear lever 120 and itsassociated selector shaft 112 is the same as the connection between gearlever 121 and its associated selector shaft 113, only the connection oflever 120 with shaft 112 will be described in detail. Correspondingcomponents in the connection of gear lever 121 with shaft 113 will bedenoted by the same reference numberals with the suffix `a` added.

As can be seen from FIG. 5, the inner end of selector shaft 112 carriesa finger 122 which is engageable either in a cut-out 123 formed in a web124, (FIG. 3) connected with selector folk 102 or in a cut out 125formed in a web 126 which is connected with selector fork 101.

Also associated with the finger 122 is a generally C-shaped interlockmember 127, which is pivotly mounted on shift rail 105 and is best seenin FIG. 8. With the finger 122 in the position shown in FIGS. 3 and 8,turning of the selector shaft 122 about its longitudinal axis willresult in displacement of the end of finger 122 which engages cutout 123in the direction of arrow V1 or V2 of FIG. 3, depending on the directionof turning of shaft 112. This in turn causes a corresponding axialshifting of selector fork 102 with a corresponding change in theoperative ratio of the main change speed set X.

It will be understood that with the finger 122 engaged in cut-out 123,end 127a of interlock member 127 engages in cut-out 125, thus preventingany axial movement of the selector fork 101.

Finger 122 is engaged in cut-out 125 to operate selector fork 101 byaxially shifting the selector shaft 112 in the direction W1 of FIG. 5.This movement of Finger 122 causes the finger to contact end 127a ofinterlock member 127 thus pivoting the interlock member about rail 105to bring end 127b into cut-out 123, as shown in FIG. 9, thus preventingmovement of selector fork 102. With finger 122 engaged in cut-out 125,selector fork 101 can be moved to again changed the operative ratio ofthe main change of speed set X.

It will be appreciated from the above that the four operative ratios ofthe main change speed set X are obtained by movements V1 and V2 ofselector finger 122 when engaged in cut-out 123 or 125, and that theselector finger is moved between the cut-outs 123 and 125 as a result ofaxial displacement of the selector shaft 112 in directions W1 and W2.

Displacements V1, V2, W1 and W2 of finger 122 are achieved as follows:

The outer end of selector shaft 112 carries a fork member 128 which issecured to shaft 112 by a set screw 129. Shaft 112 is supported by asurrounding extension 130 which is formed as an integral part of a plate131, which closes an access opening 132 in the side of the transmissionhousing. Fork member 128 is connected via a pin 133 with a lower levermember 134, whose lower end is pivotly mounted by a ball mount 135 in asupport bracket 136 which is bolted to plate 131. The upper end of lever134 is provided with a part spherical member 137 which is received in acup member 138 provided on the lower end of gear lever 120. Gear lever120 is pivoted intermediate its ends on a part-spherical ball 139 whichsupported via a part-spherical cup-like mount 250 which is secured tothe floor 251 of a tractor cab or operators platform. As is conventionalpractice the cab or platform is itself also vibrationally isolated fromthe tractor chassis which includes the transmission housing.

FIG. 28 shows an alternative form of mount for levers 120 and 121 inwhich the ball 139 and mount 250 are replaced by the vibration isolatingmount 400 having a central metal tube 491 through which lever 120extends and is pinned to the lever at 402. The tube 401 is surrounded byan annulus of polymeric material 403 which is bonded to the tube 401 andalso to an outer metal ring 404 which is then secured to the cab floor251. The necessary pivotal articulation of lever 120 is accommodated byflexing of the polymeric annulus 403.

The ratio selector mechanism described above in which gear lever 120 ismounted above the lower lever 134 which in turn moves a selector shaft112 is the subject of the Applicant's co-pending U.S. patent applicationSer. No. 672,329 (which claims priority from UK Patent Application No.8307097).

FIG. 12 shows the gate pattern for the movement of gear lever 120 whileFIGS. 10 and 11 diagramatically shown the various movements of the gearlever 120 and the corresponding movements of the selector finger 122.

If we assume that the operator wishes to select the first ratio he movesthe lever 120 to the left from its neutral position NE shown in FIG. 12and then forwardly. These two movements are indicated by positions 120'and 120" in FIGS. 10 and 11 respectively. As can be seen from FIG. 10sideways movement of the lever 120 to the position 120' results in thepivoting to the left of the lower lever 134 with consequence axialsliding of selector shaft 112 in direction W1 to bring finger 122 intothe cut-out 125 associated with selector fork 101. Subsequent forwardmovement of the lever 120 to the position 120" showed in FIG. 11 turnsselector shaft 112 in order to displace the selector fork 101 indirection V2 in order to select the first ratio.

As will be appreciated from the above description, all four ratios ofthe name change speed set X can be selected using the combination ofsideways and forward/rearward movement of lever 120.

As can be seen from FIG. 12, an additional position designated H/L forthe selection of the High/Low range of gear set Y is provided to theleft of the first and second ratio positions of the gear lever gate.

As can be seen from FIG. 5, selector shaft 112 is provided with a crosspin 140 against which bears a ring 141 which is spring biased by a coilspring 142 away from selector finger 122. When the gear lever 120 ismoving in the first and second ratio plane indicated as `m` in FIG. 12(towards which the selector mechanism is biassed by spring means notshown), the cross pin 140 is at location 140' in FIG. 5 (i.e., just tothe left of the inside of cover 131). If the lever 120 is in the neutralplane indicated at `n` in FIG. 12, the cross pin 140 is able to enter aslot 143 provided in the cover 131, thus allowing the selector shaft 112to move further to the right in the direction W1. This further movementto the right is arranged to operate an electrical switch 144 shown inFIG. 6 which changes the state of the solenoid valve which controls theconnection and disconnection of inlet 118 with the hydraulic pressuresupply. Thus, for example, a movement of lever 120 to the left past theplane `m` will select the High range of gear set Y, and the nextmovement of the lever to the left past the plane `m` will select thelower range of gear set Y, etc. This is a simple and efficient manner toincorporate yet another function under the control lever 120.

FIG. 13 shows the gate for gear lever 121 which controls the input gearZ which provides two forward and two reverse range ratios as describedabove. As will be evident from the earlier description, the provision ofthe two speed output gear set Y which is controlled by lever 120 doublesthe number of ranges provided by the transmission to four forward rangesFA, FB, FC and FD and four reveres ranges RA, RB, RC and RD.

This doubling of the avialable ranges is reflected in the gate for lever121 in which when the lever 121 is moved forwardly either forward rangeFA or FC is engaged depending on whether the output gear set Y is in itslower or higher range. In a similar fashion, when lever 121 is movedrearwardly either forward range FB or FD is engaged depending on whethergear set Y is in its lower or higher range.

As indicated by FIG. 13, reverse ranges RA and RC can be engaged bymoving lever 121 sideways and rearwardly from the FA/FC position whilereverse ranges RB and RD can be engaged by moving lever 121 sideways andforwardly from the FB/FD position (RA and RB are engaged with gear set Yin its lower range and RC and RD are engagerd with gear set Y is in itshigher range).

As can be seen from FIG. 7, the inner end of selector shaft 113 isprovided with an arm 150 and a projection 151. Projection 151 is engagedin a cutout 152 in a selector member 153 (see FIG. 14) which is mountedon a selector rod 154 which carries a selector fork 155 which operatessleeve M. Projection 151 is also engageable in a cutout 156 in aselector member 157 which is bolted to the selector fork 100 whichoperates sleeve N.

As previously indicated, forward range FA is provided when both sleevesM and N are moved to the right of their central positions as viewed inFIG. 1 and gear set Y is in its lower range. This range is selected whenthe gear lever 121 occupies the FA/FC position of FIG. 13 in which theprojection 151 is in engagement with both cutouts 152 and 156 as shownin FIG. 15 and diagrammatically illustrated in FIG. 17.

If the operator now wishes to change to the second forward range FB ofinput gear set Z (in which both sleeves M and N are to the left of theircontrol positions as viewed in FIG. 1 and gear set Y is in its lowerrange) he moves the gear lever 121 to the FB position of FIG. 13 whichmaintains the projection 151 in cut outs 152 and 156 and moves theselector members 153 and 157 to the FIG. 16 position by rotation ofshaft 113. FIGS. 20, 21 and 23 diagrammatically illustrate theconfiguration of gear lever 121, lever 134a and shaft 113 when forwardrange FA, FB, FC and FD are selected. It will be observed that duringmovement between the FA and FB positions of FIGS. 17 and 16 flange 165on arm 150 remains to the left of baulking plate 166. Baulking plate 166prevents axial movement of selector shaft 113 in direction W3 atlocations intermediate the FA and FB positions. Preferably the selectormechanism is biased so as to tend to move the gear lever 121 to theforward range change plane FA/FC to FB/FD of FIG. 13. This can beachieved, for example, by a spring 305 which acts between fork member128a and lever member 134a and tends to bias fork member 128a towardsand into contact with the end 306 of extension 130a of FIG. 7 in whichposition projection 151 engages both cut outs 152 and 156.

If with the transmission in forward range FA (see FIG. 17) the operatornow wishes to select reverse range RA the selector shaft 113 is movedaxially in direction W3 (see FIG. 18) to disengage cut out 156 inselector member 157 by sideways pivotting of lever 121 to the FIG. 22position. The selector shaft is then turned by rearward movement of gearlever 121 to the FIG. 21 position to bring the selector member 153 tothe dotted line RA position of FIG. 18 and thus slide sleeve M to theleft of its central position to complete the selection of reverse ratioRA. This movement of lever 121 from forward range ratio FA to reverserange ratio RA is shown by dotted line 200 in FIG. 13. When projection151 is in the RA position of FIG. 18, movement of the projection to theleft (direction W4) into the path of movement of selector member 157 isprevented by ear 160 provided on selector member 157. During turning ofselector shaft 113 to the RA position of FIG. 18 the flange 165 movesunder the baulking plate 166.

In a similar fashion if the input gear set Z is in forward range FB (seeFIG. 16) the reverse range RB (see FIG. 19) is achieved by axiallydisplacing selector shaft 113 in direction W3 to disengage cut-out 156by sideways pivotting the gear lever 121 to the FIG. 22 position andthen turning the selector shaft by forward pivotting of gear lever 121to the FIG. 23 position. This will bring the selector member 153 to thedotted-line RB position of FIG. 19 and thus moves the sleeve M to theright of its central position to complete the selection of reverse ratioRB. Again when projection 151 is in the RB position of FIG. 19 movementof projection 151 to the left (direction W4) is prevented by ear 170provided on selector member 157. During turning of shaft 113 to the RBposition of FIG. 19 the flange 165 moves under the baulking plate 166.The movement of lever 121 from the forward range FB to reverse range RBis shown by dotted line 201 in FIG. 13.

As will be evident from the above description the interaction betweenflange 165 and baulking plate 166 and the ears 160 and 170 fully definethe operating gate of lever 121 so that there is no additionalrequirement to physcially guide the lever 121.

FIGS. 26 and 27 show a modified form of gear lever selector mechanismfor use in connection with gear lever 120 in which those components ofequivalent function to those described above in relation to FIGS. 5 and6 are similarly numbered.

In the arrangement shown in FIGS. 26 and 27 the electrical switch 144has been moved adjacent the path of movement of interlock member 127 anda spring 300 is provided which acts between lever 134 and fork member128 and in conjunction with spring 142 acts to bias shaft 112 and hencefinger 122 to the position shown in FIG. 26 in which gear lever 120 isin the first and second ratio in plane m of FIG. 12. In order to operateswitch 144 to change between the higher and lower ranges of gear set Y,the lever 120 is moved whilst in the neutral plane n to the left ofplane m to the H/L position of FIG. 12. This draws shaft 112 and finger122 to the right of the position shown in FIG. 26 against the action ofspring 142 and pivots interlock member 127 clockwise to dotted-lineposition 127' to contact and operate switch 144. As in the previouslydescribed arrangement each movement of gear lever 120 to the H/Lposition of FIG. 12 causes gear set Y to change between is higher andlower ranges.

A guidance plate 301 with protruding flanges 302 is secured to theinside of plate 131. This guidance plate ensures that FIG. 122 can onlymove to the right of the position shown in FIG. 26 when in the neutralplane n.

One of the main benefits of the present invention is that the removal ofthe lower selector forks 100, 101, 102, and 103 can be easily effectedvia the opening 132 by removing set screw 108 and then sliding theselector fork mounting rail 105 to the right as viewed in FIG. 3 whilstremoving the forks one by one from the left-hand end of the rail 105.

It will also be appreciated that a particularly economic selector forkarrangement is also provided particularly when the selector fork 104 forthe creeper unit C and the hydraulic cylinder 114 for the operation ofselector fork 103 are also mounted on the same rail 105.

I claim:
 1. A vehicle transmission having a housing a multi-ratio mainchange speed gear set and a first range change gear set in series withinthe housing, selector forks for effecting speed and range changes in thetransmission, a single selector fork rail on which the selector forksare mounted for axial sliding movement, rail support means and railfastening means for supporting and fastening the rail in a stationaryposition within the housing and also supporting the rail for axialsliding within the housing on release of the fastening means, and anaccess aperture in the housing whereby, after release of the railfastening means, the selector forks can be removed one by one from oneend of the rail via the aperture on sliding of the rail within thehousing.
 2. A transmission according to claim 1 having a second rangechange gear set operated by a further selector fork which is alsomounted on and movable relative to the same selector fork rail.
 3. Atransmission according to claim 1 having a creeper gear facility whichis operated by a still further selector fork which is also mounted onand movable relative to the same selector fork rail.
 4. A transmissionaccording to claim 1 having an hydraulic cylinder which surrounds partof the rail, the cylinder being connected with one of the range changeselector forks for axial sliding of said fork relative to the rail.
 5. Atransmission according to claim 4 in which the hydraulic cylinder isdouble-acting and contains a differential area double-acting pistonwhich surround the rail and divides the cylinder into two operatingchambers, the arrangement being such that if both operating chambers aresubjected to the same level of pressure the piston moves in a firstdirection and if only one chamber is pressurised the piston moves in theopposite direction.
 6. A transmission according to claim 1 having aninterlock member which ensures the movement of only one of the selectorforks of the main change speed gear set at a given time, the interlockmember being pivotally mounted on the same selector fork rail.
 7. Atransmission according to claim 1 in which a pair of the selector forksare moved relative to the selector fork rail by a selector mechanismcomprising an upper manually operable gear lever which is mountedintermediate its ends for pivotting about two perpendicular axes, alower lever member which is pivotted in a fixed mount at its lower endand is operatively connected at its upper end with the lower end of thegear lever, the lower lever being interconnected intermediate its endswith a ratio selector shaft whose longitudinal axis passes through thefixed mount, the arrangement being such that pivotting of the gear leverabout one of the perpedicular axes causes axial displacement of theselector shaft in a direction parallel to its longitudinal axis toengage one or both of said pair of selector forks and pivotting of thegear lever about the other perpendicular axis causes turning of theselector shaft about its longitudinal axis to move the engaged selectorfork or forks relative to the selector fork rail.